1. Field of the Invention
This invention relates to an automatic document feeder, and more particularly to a duplex automatic document feeder.
2. Description of the Related Art
Referring to FIG. 1, a compact auto-document feeder 1 disclosed in U.S. Pat. No. 5,784,680 includes a document setting tray 11, a scanning module 12, an ejecting tray 13, and a switching gate 14. A first transfer path 101 extends from the document setting tray 11 to the scanning module 12. A second transfer path 102 extends from the scanning module 12 to the ejecting tray 13. A third transfer path 103 extends from a downstream end of the second transfer path 102 to the first transfer path 101. The switching gate 14 is disposed among the second, third, and fourth transfer paths 102, 103, 104. Operations of the feeder 1 performed on one document will be described hereinafter.
When the feeder 1 is operated in a simplex document-feeding mode, the document is fed from the document-setting tray 11, and is moved through the first transfer path 101 and the scanning module 12 for scanning of the document. Subsequently, the document is moved onto the ejecting tray 13 through the second transfer path 102 with the switching gate 14 being disposed in a first position shown by the solid lines in FIG. 1.
When the feeder 1 is operated in a duplex document-feeding mode, the document is moved from the first transfer path 101 onto the second transfer path 102 for scanning of a first side surface thereof. At the same time, the switching gate 14 is pivoted to a second position shown by the phantom lines in FIG. 1. Hence, the document is moved from the second transfer path 102 onto the fourth transfer path 104 and a roller unit 16. Thereafter, the rotational directions of rollers of the roller unit 16 are changed so as to return the document to the first transfer path 101 via the third transfer path 103. Thus, the document is moved from the first transfer path 101 onto the second transfer pat 102 once again so as to allow for scanning of a second side surface thereof. After completion of the scanning of the first and second side surfaces of the document, in order to enable ejection of the document with the first side surface of the document facing downwardly, the document must be further moved through a document-inverting path defined by the third transfer path 103, a downstream portion of the first transfer path 101, and the second transfer path 102 so as to drop from the downstream end of the second transfer path 102 onto the ejecting tray 13.
The feeder 1 has the following disadvantages:    (1) The document-inverting path is long, thereby reducing the operating efficiency of the feeder 1.    (2) Referring to FIG. 2, before a trailing end 151 of the document is moved from the roller unit 16, a leading end 152 of the document may be moved onto the roller unit 16, thereby resulting in occurrence of a paper jam.
Referring to FIG. 3, to solve the paper jam problem, a duplex automatic document feeder 2 disclosed in US Patent Application publication no. 2002/0054382 includes first, second, third, and fourth document-feeding paths 201, 202, 203, 204, a scanning module 22, a middle roller 23, an upper roller 24, and a lower roller 25. Relative positions among the first, second, and third document-feeding paths 201, 202, 203, as well as the scanning module 22 are similar to those of U.S. Pat. No. 5,784,680. The fourth document-feeding path 204 is U-shaped, and has upper and lower horizontal sections 205, 206, and a curved section 207. The middle roller 23 is disposed between the upper and lower horizontal sections 205, 206. The upper and lower rollers 24, 25 are in frictional contact with the middle roller 23, and are respectively adjacent to the upper and lower horizontal sections 205, 206 of the fourth document-feeding path 204. After completion of a second scanning operation, the document is moved from the second document-feeding path 202 onto the lower horizontal section 206 of the fourth document-feeding path 204. When the document is moved into a space between the middle roller 23 and the lower roller 25, the rotational directions of the middle roller 23 and the lower roller 25 are changed so as to allow the document to be moved onto the curved section 207 and the upper horizontal section 205 of the fourth document-feeding path 204, thereby passing through a space between the middle roller 23 and the upper roller 24. Finally, the document is moved from the middle roller 23 and the upper roller 24 onto a discharging tray (not shown).
The lower roller 25 is movable relative to the middle roller 23 between upper and lower positions shown respectively by the solid and phantom lines in FIG. 3 through operation of an elevating mechanism (not shown). When the lower roller 25 is moved to the lower position, two documents can be moved in two opposite directions through the space between the middle roller 23 and the lower roller 25, thereby preventing occurrence of a paper jam.
However, the structure of the feeder 2 is complex due to the presence of the elevating mechanism. This increases the manufacturing costs of the feeder 2.